1. Field of the Invention
This invention relates generally to footwear, and more particularly to an article of footwear having a system for providing cushioning and support for the comfort of the wearer.
2. Related Art
One of the problems associated with shoes has always been striking a balance between support and cushioning. Throughout the course of an average day, the feet and legs of an individual are subjected to substantial impact forces. Running, jumping, walking and even standing exert forces upon the feet and legs of an individual which can lead to soreness, fatigue, and injury.
The human foot is a complex and remarkable piece of machinery, capable of withstanding and dissipating many impact forces. The natural padding of fat at the heel and forefoot, as well as the flexibility of the arch, help to cushion the foot. An athlete""s stride is partly the result of energy which is stored in the flexible tissues of the foot. For example, during a typical walking or running stride, the achilles tendon and the arch stretch and contract, storing energy in the tendons and ligaments. When the restrictive pressure on these elements is released, the stored energy is also released, thereby reducing the burden which must be assumed by the muscles.
Although the human foot possesses natural cushioning and rebounding characteristics, the foot alone is incapable of effectively overcoming many of the forces encountered during athletic activity. Unless an individual is wearing shoes which provide proper cushioning and support, the soreness and fatigue associated with athletic activity is more acute, and its onset accelerated. This results in discomfort for the wearer which diminishes the incentive for further athletic activity. Equally important, inadequately cushioned footwear can lead to injuries such as blisters, muscle, tendon and ligament damage, and bone stress fractures. Improper footwear can also lead to other ailments, including back pain.
Proper footwear should complement the natural functionality of the foot, in part by incorporating a sole (typically, an outsole, midsole and insole) which absorbs shocks. However, the sole should also possess enough resiliency to prevent the sole from being xe2x80x9cmushyxe2x80x9d or xe2x80x9ccollapsing,xe2x80x9d thereby unduly draining the energy of the wearer.
In light of the above, numerous attempts have been made over the years to incorporate into a shoe means for providing improved cushioning and resiliency to the shoe. For example, attempts have been made to enhance the natural elasticity and energy return of the foot by providing shoes with soles which store energy during compression and return energy during expansion. These attempts have included using compounds such as ethylene vinyl acetate (EVA) or polyurethane (PU) to form midsoles. However, foams such as EVA tend to break down over time, thereby losing their resiliency.
Another concept practiced in the footwear industry to improve cushioning and energy return has been the use of fluid-filled devices within shoes. These devices attempt to enhance cushioning and energy return by utilizing cushions containing pressurized fluid that are disposed adjacent the heel and forefoot areas of a shoe. The overriding problem of these devices is that the cushioning means are inflated with a pressurized gas which is forced into the cushioning means, usually through a valve accessible from the exterior of the shoe.
There are several difficulties associated with using a pressurized fluid within a cushioning device. Most notably, it may be inconvenient and tedious to constantly adjust the pressure or introduce a fluid to the cushioning device. Moreover, it is difficult to provide a consistent pressure within the device thereby giving a consistent performance of the shoes. In addition, a cushioning device which is capable of holding pressurized gas is comparatively expensive to manufacture. Further, pressurized gas tends to escape from such a cushioning device, requiring the introduction of additional gas. Finally, a valve which is visible to the exterior of the shoe negatively affects the aesthetics of the shoe, and increases the probability of the valve being damaged when the shoe is worn.
A cushioning device which, when unloaded contains air at ambient pressure, provides several benefits over similar devices containing pressurized fluid. For example, generally a cushioning device which contains air at ambient pressure will not leak and lose air, because there is no pressure gradient in the resting state. The problem with many of these cushioning devices is that they are either too hard or too soft. A resilient member that is too hard may provide adequate support when exerting pressure on the member, such as when running. However, the resilient member will likely feel uncomfortable to the wearer when no force is exerted on the member, such as when standing. A resilient member that is too soft may feel comfortable to a wearer when no force is exerted on the member, such as when standing or during casual walking. However, the member will likely not provide the necessary support when force is exerted on the member, such as when running. Further, a resilient member that is too soft may actually drain energy from the wearer.
A shoe which incorporates a cushioning system including a means to provide resilient support to the wearer during fast walking and running, and to provide adequate cushioning to the wearer during standing and casual walking is disclosed in U.S. Pat. No. 5,771,606 to Litchfield et al., which is incorporated herein in its entirety by reference. U.S. Pat. No. 5,771,606 describes a resilient insert member including a plurality of heel chambers, a plurality of forefoot chambers and a central connecting passage fluidly interconnecting the chambers. The resilient insert is made from an elastomeric material and may contain air at ambient pressure. The resilient insert is placed between an outsole and a midsole of an article of footwear.
Although the resilient insert of U.S. Pat. No. 5,771,606 provides resilient support and adequate cushioning to the wearer during a wide range of activities, the arrangement and shape of the forefoot chambers results in a decrease in flexibility of the resilient insert about the metatarsal area of the foot. In addition, the shape, interconnection and placement of the heel chambers make the resilient insert somewhat rigid, such that substantial cushioning only occurs as to downward forces during heel strike.
Accordingly, what is needed is a shoe which incorporates a cushioning system including a means to provide resilient support and adequate cushioning to the wearer that anatomically compliments the wearer""s foot so that flexibility is maintained and stability increased. In addition, the cushioning system must be more compliant during a wearer""s gait thereby providing maximum support and cushioning benefit when downward and/or shear forces are applied.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the article of footwear of the present invention comprises a sole and a resilient support and cushioning system. The system of the present invention includes a resilient insert member and a bladder disposed within an article of footwear.
In one embodiment, the resilient insert includes a plurality of heel chambers, a plurality of forefoot chambers and a central connecting passage fluidly interconnecting one of the heel chambers with one of the forefoot chambers. The forefoot chambers are staggered and fluidly interconnected in series along either side of forefoot chamber interconnection passages. Each forefoot chamber is arranged so that a line taken lengthwise through each chamber is essentially perpendicular to a longitudinal centerline of the resilient insert.
The single central connecting passage and arrangement of the forefoot chambers i.e., their length extending in a lateral rather than a longitudinal direction, allow for a relatively xe2x80x9cfree spacexe2x80x9d below the metatarsal area of the foot allowing for better flexibility. Further, the staggered arrangement of the forefoot chambers on either side of the centrally located forefoot chamber interconnection passages reduces the number of hard edges of the forefoot chamber under the metatarsal region of the foot thereby reducing the rigidity of the insert in that area while still maintaining its supportive function.
In one embodiment, the central connecting passage contains an impedance means to restrict the flow of air between the heel chambers and the forefoot chambers. During heel strike, the impedance means prevents air from rushing out of the heel chambers too quickly. Thus, the air in the heel chambers provides support and cushioning to the wearer""s foot during heel strike.
The bladder of the present invention includes at least one heel chamber, at least one forefoot chamber and at least one connecting passage fluidly interconnecting the two chambers. In one embodiment, the bladder is disposed above the midsole of the article of footwear, and provides cushioning to the wearer""s foot. In one embodiment, the bladder is vacuum formed from two sheets of resilient, non-permeable elastomeric material such that the bladder contains air at slightly above ambient pressure.
In use, the bladder provides cushioning to the wearer""s foot while standing or during casual walking. The resilient insert provides added support and cushioning to the wearer""s foot during fast walking and running. In an alternate embodiment, for example, for use as a high performance shoe, the article of footwear may contain only the resilient insert disposed in the sole. In another alternate embodiment, for example, for use as a casual shoe, the article of footwear may contain only the bladder disposed above the sole.
When stationary, the foot of a wearer is cushioned by the bladder. When the wearer begins a stride, the heel of the wearer""s foot typically impacts the ground first. At this time, the weight of the wearer applies downward pressure on the heel portion of the resilient insert, causing the heel chambers to be forced downwardly. A large lateral heel chamber absorbs the main impact. In addition, the wearer""s forward momentum at foot strike causes the heel of the foot to move forward briefly while a heel portion of the shoe sole is still in contact with the ground. This forward velocity creates a shear load which forces a decoupled portion of the large lateral heel chamber to flex forward with the weight of the wearer. As the foot of the wearer then rolls medially and forwardly, the forces on the heel chambers dissipate. With reference to the large lateral heel chamber, this results in the decoupled portion returning to its original unflexed position. The fore-aft flexing of the large lateral heel chamber acts as a shock absorber in the longitudinal direction of the insert due to the shearing action within the chamber that allows the foot to briefly xe2x80x9cglidexe2x80x9d forward and aft upon the resilient insert.
In this embodiment, the heel chambers are also fluidly interconnected in series in a U-shape along heel chamber interconnection passages. Each heel chamber has a functionally distinctive shape with the large lateral heel chamber having a decoupled portion capable of fore-aft flexing. The fore-aft flexing of the large lateral heel chamber creates a shearing action within that chamber which allows for cushioning of shear forces as well as cushioning of downward forces. The rearmost medial heel chamber also has a decoupled portion which acts to supply air to a forward triangular-shaped heel chamber on the medial side of the resilient insert. The triangular-shaped heel chamber traps air and acts as a medial post to help prevent over-pronation of the foot.
The heel chambers of the resilient insert are connected via heel chamber interconnection passages. A rearmost passage essentially divides the heel portion into a medial region and a lateral region so that the two regions act independently of each other. The medial region heel chambers are designed geometrically to help compensate for the problem of over-pronation, the natural tendency of the foot to roll inwardly after heel impact.
During a typical gait cycle, the main distribution of forces on the foot begins adjacent the lateral side of the heel during the xe2x80x9cheel strikexe2x80x9d phase of the gait, moves toward the center axis of the foot in the arch area at mid-stride, rolls medially and then moves to the medial side of the forefoot area during xe2x80x9ctoe-off.xe2x80x9d
The configuration of the heel chamber interconnection passage between the rearmost medial heel chamber and the triangular-shaped medial heel chamber ensures that the air flow within the resilient insert complements such a gait cycle. Thus, the downward pressure resulting from heel strike causes air within the resilient insert to flow from the lateral region into the medial region, increasing air pressure therein. The medial region stiffens due to the increased air pressure, thereby providing support to the medial region of the wearer""s foot and inhibiting over-pronation. Compression of the heel portion also causes the air in the lateral region to be forced forwardly, through the central connecting passage and into the forefoot portion of the resilient insert.
In addition, the forefoot and heel chambers of the resilient insert have substantially concave upper surfaces which extend beyond each side of the wearer""s foot and act to cradle the foot upon impact thereby improving stability. The resilient insert is preferably blow molded from an elastomeric material, and may contain air at ambient pressure or slightly above ambient pressure. The resilient insert is disposed in the sole of an article of footwear.
The flow of air into the forefoot portion causes the forefoot chambers to expand, which slightly raises the forefoot or metatarsal area of the foot. When the forefoot of the wearer is placed upon the ground, the expanded forefoot chambers help cushion the corresponding impact forces. As the weight of the wearer is applied to the forefoot, the downward pressure caused by the impact forces causes portions of the forefoot chambers to compress while their concave upper surfaces inflate to cradle the foot and increase stability. Simultaneously, air is thrust rearwardly through the central connecting passage into the heel portion.
After xe2x80x9ctoe-off,xe2x80x9d no downward pressure is being applied to the article of footwear, so the air within the resilient insert should return to its normal state. Upon the next heel strike, the process is repeated.
In light of the foregoing, it will be understood that the system of the present invention provides a variable, non-static cushioning, in that the flow of air within the bladder and the resilient insert complements the natural biodynamics of an individual""s gait.